FBAR devices that incorporate one or more film bulk acoustic resonators (FBARs) form part of an ever-widening variety of electronic products, especially wireless products. For example, modern cellular telephones incorporate a duplexer in which each of the band-pass filters includes a ladder circuit in which each element of the ladder circuit is an FBAR. A duplexer incorporating FBARs is disclosed by Bradley et al. in U.S. Pat. No. 6,262,637 entitled Duplexer Incorporating Thin-film Bulk Acoustic Resonators (FBARs), assigned to the assignee of this disclosure and incorporated into this disclosure by reference. Such duplexer is composed of a transmitter band-pass filter connected in series between the output of the transmitter and the antenna and a receiver band-pass filter connected in series with 90° phase-shifter between the antenna and the input of the receiver. The center frequencies of the pass-bands of the transmitter band-pass filter and the receiver band-pass filter are offset from one another. Ladder filters based on FBARs are also used in other applications.
FIG. 1 shows an exemplary embodiment of an FBAR-based band-pass filter 10 suitable for use as the transmitter band-pass filter of a duplexer. The transmitter band-pass filter is composed of series FBARs 12 and shunt FBARs 14 connected in a ladder circuit. Series FBARs 12 have a higher resonant frequency than shunt FBARs 14.
FIG. 2 shows an exemplary embodiment 30 of an FBAR. FBAR 30 is composed a pair of electrodes 32 and 34 and a piezoelectric element 36 between the electrodes. The piezoelectric element and electrodes are suspended over a cavity 44 defined in a substrate 42. This way of suspending the FBAR allows the FBAR to resonate mechanically in response to an electrical signal applied between the electrodes.
United States patent application publication nos. 2005 0 093 654 and 2005 0 093 658, assigned to the assignee of this disclosure and incorporated by reference, disclose a band-pass filter that incorporates a decoupled stacked bulk acoustic resonator (DSBAR) composed of a lower FBAR, an upper FBAR stacked on lower FBAR and an acoustic decoupler between the FBARs. Each of the FBARs is composed of a pair of electrodes and a piezoelectric element between the electrodes. An electrical input signal is applied between electrodes of the lower FBAR and the upper FBAR provides a band-pass filtered electrical output signal between its electrodes. The electrical input signal may alternatively be applied between the electrodes of the upper FBAR, in which case, the electrical output signal is taken from the electrodes of the lower FBAR. Band-pass filters composed of two of the above-described band-pass filters connected in series are described in United States patent application publication no. 2005 0 140 466.
United States patent application publication nos. 2005 0 093 655 and 2005 0 093 656, assigned to the assignee of this disclosure and incorporated by reference, disclose a film acoustically-coupled transformer (FACT) composed of two decoupled stacked bulk acoustic resonators (DSBARs). A first electrical circuit interconnects the lower FBARs of the DSBARs in series or in parallel. A second electrical circuit interconnects the upper FBARs of the DSBARs in series or in parallel. Balanced or unbalanced FACT embodiments having impedance transformation ratios of 1:1 or 1:4 can be obtained, depending on the configurations of the electrical circuits. Such FACTs also provide galvanic isolation between the first electrical circuit and the second electrical circuit.
The FBAR described above with reference to FIG. 2 and devices, such as ladder filters, DSBARs, band-pass filters and FACTs, incorporating one or more FBARs will be referred to generically in this disclosure as FBAR devices.
Most FBAR devices have a frequency response having a band pass characteristic characterized by a center frequency. The constituent FBARs have a frequency response characteristic characterized by a resonant frequency. In practical embodiments of current FBAR devices in which the material of the piezoelectric element is aluminum nitride (AIN) and the material of the electrodes is molybdenum (Mo), the resonant frequency of the FBAR(s) has a temperature coefficient from about −20 ppm/° C. to about −35 ppm/° C. Such temperature coefficients reduce the temperature range over which the FBAR device can meet its pass bandwidth specification. Such temperature coefficients additionally reduce manufacturing yield, because the bandwidth limits to which the FBAR devices are tested have to be inset to ensure that the FBAR device will meet its bandwidth specification over its entire operating temperature range.
Practical embodiments of the above-described FBAR devices are fabricated suspended over a cavity defined in a substrate. To provide a plane surface on which to fabricate the FBAR device, the cavity is filled with sacrificial material near the beginning of the fabrication process. After the FBAR device has been fabricated, the sacrificial material is removed, leaving the FBAR device suspended over the cavity. A typical sacrificial material is phosphosilicate glass, and the sacrificial material is removed from the cavity by a wet etch process that uses hydrofluoric acid (HF) as an etchant. Since the release etch is performed towards the end of the fabrication process, the materials of the FBAR device have to be etch compatible with HF.
What is needed, therefore, is an FBAR device whose resonant frequency has a reduced temperature coefficient and that can be fabricated using materials that are etch compatible with the release etch.